The present invention relates to an internal combustion engine ignition timing control system with knock control which functions such that the occurrence of knocking is detected from the vibrations or sound produced outside the engine cylinders due to the cylinder pressure and the ignition timing of the engine is retarded in accordance with the detected knocking.
Recently, a variety of so-called knock feedback systems have been under study in which the ignition timing of an engine is retarded by detecting the occurrence of knocking in the engine. These systems are generally designed as follows. In other words, the vibrations or sound produced outside the engine cylinders due to the cylinder pressure is detected so that when the vibrations or sound exceeds a preset level (a knocking determination level), the occurrence of knocking is determined and a knock signal is generated. The ignition timing is retarded when the knock signal is generated and the ignition timing is advanced when there is no knock signal, thereby always controlling the ignition timing at around the knock limiting value and improving the fuel consumption and power output performance of the engine.
In this type of knock feedback system, the degrees of crank angle at the ignition is retarded upon detection of knocking or the amount of ignition timing retard per knocking is preliminarily determined and it is usually on the order of 1.degree. crank angle. This amount of retard per knocking event is the most important factor associated with the control performance of the ignition timing. This amount of retard will now be described with reference to FIG. 1. In FIG. 1, (a) and (b) show the ignition timing variations during the steady-state operation and (c) shows the ignition timing response characteristic during the transitional period. In each of (a) to (c) of FIG. 1, the abscissa represents the time and the ordinate represents the ignition timing.
In the Figure, (a) shows the ignition timing variation during the steady-state operation where the amount of retard per knocking event is large (e.g., 2.degree. crank angle), and (b) shows the ignition timing variation during the steady-state operation where the amount of retard per knocking event is small (e.g., 0.5.degree. crank angle). In each of (a) and (b), the dot-and-dash line indicates the desired ignition timing for the ignition timing control and it usually corresponds to the trace knock limit ignition timing. This ignition timing maintains the engine at the proper level of knocking and improves the fuel consumption. A glance at a comparison between (a) and (b) clearly shows that the control performance during the steady-state operation is better when the amount of retard per knocking is small ((b) of FIG. 1) than when it is large. The reason is that the deviation from the desired ignition timing increases when the amount of retard is large ((a) of FIG. 2) so that the knocking sound increases when the ignition timing deviation is on the advance side and loss occurs in the power output and fuel consumption when the timing deviation is on the retard side. Of course, if the amount of retard per knocking is reduced excessively, the knocking sound suppressing effect is deteriorated and thus the amount of retard during the steady-state operation should preferably be in the range of 0.3.degree. to 0.5.degree. crank angle. As just described, during the steady-state operation the ignition timing control performance is improved with a decrease in the amount of retard per knocking event.
However, the reverse can be said to be true in the case of the ignition timing response characteristic during the transitional period. In (c) of FIG. 1 showing the ignition timing response characteristic during the transitional period (rapid acceleration period), r.sub.1 designates a case where the amount of retard is small (e.g., 0.5.degree. crank angle) and r.sub.2 a case where the amount of retard is large (e.g., 2.degree. crank angle). It will be seen from the Figure that where the amount of retard per knocking event is small (i.e., the case 1), the ignition timing response characteristic is deteriorated and thus knocking occurs continuously during the transitional period, e.g., the rapid acceleration period thereby causing a feeling of unpleasantness on the part of the driver and causing damage to the engine. Therefore, the amount of retard per knocking event must be increased during the transional period. It will thus be seen that in order to improve both the ignition timing control performance during the steady-state period and the ignition timing response characteristic during the transitional period, it is absolutely necessary to distinguish between the steady-state period and the transitional period and changes over the amount of retard per knocking event from one value to another.
In the past, however, there has been no means capable of accurately distinguishing between the steady-state period and the transitional period and it has been impossible to change over the amount of retard per knocking event between the two values. For instance, in the case of a method which detects the acceleration operation of an engine and changes over the amount of retard correspondingly, there is a case where only slight knocking occurs even during the acceleration operation depending on the variations in characteristics among different engines, the surrounding conditions, etc., and in such case the large preset amount of retard frequently results in an excessively large retard angle thereby deteriorating the acceleration performance. As a result, the conventional preset amount of retard (e.g., 1.degree. crank angle) has represented a compromise between those for the steady-state and transitional periods and the engine performance has been unavoidably deteriorated during both the steady-state period and the transitional period.